Lentiviruses, a subfamily of retroviruses that cause slow viral diseases in domestic animals, are being intensively analyzed by immunological techinques and by molecular cloning technology. We have recently obtained molecular clones of equine infectious anemia virus (EIAV) and demonstrated that this virus is relatively closely related to the prototype caprine lentivirus, CAEV, previously cloned in this laboratory. Nucleotide sequence analysis of the 5' end of the pol genes of both EIAV and CAEV revealed that this highly conserved region is closely related to the corresponding region of HTLV-III, demonstrating that the causative agent of acquired immunodeficiency syndrome (AIDS) and these lentiviruses are evolved from a common progenitor. Currently, immunoassays and candidate vaccines are being developed to facilitate early diagnosis of infected animals and to determine if these important diseases (and models of AIDS) can be controlled. Analysis of the sera of individuals who handle squirrel monkeys who harbor the oncogneic Herpesvirus saimira (HVS) revealed the important finding that about 7% of this group were positive for antiboides to the virus. Analysis of these sera, and additional experiments which examined the capacity of hvs to replicate in human cells cultured in vitro, suggest that either HVS does not replicate in vivo or does so by a mechanism inconsistent with what is seen in vitro. Nevertheless, it seems prudent to suggest that the guidelines recommend for class II oncogenic viruses continue to be followed.